A structure of a carbon nanotube (hereinafter, referred to as CNT) was first found in 1991. Synthesis, physical property, and application of the carbon nanotube have been actively studied. Also, it has been confirmed that the CNT is produced by adding transition metals, such as ferrum (Fe), nickel (Ni), cobalt (Co), etc., at the time of discharging electricity. A full study started from a preparation of a significant amount of samples by a laser evaporation method in 1996. The CNT takes a form of a round wound hollow tube whose graphite surface is a diameter of a nano size. At this time, the CNT has electrical characteristics, such as conductivity, semi-conductivity, etc., according to the wound angle and structure of the graphite surface. Moreover, the CNT is divided into a single-walled carbon nanotube (SWCNT), a double-walled carbon nanotube (DWCNT), a thin multi-walled carbon nanotube, a multi-walled carbon nanotube (MWCNT), and a roped carbon nanotube according to the number of graphite walls.
In particular, the CNT has excellent mechanical strength or elastic strength, chemical stability, eco-friendliness, and electrical conductivity and semi-conductive properties as well as having an aspect ratio larger than existing materials, wherein the aspect ratio reaches about 1,000 as a diameter of 1 nm to several tens nm and a length of several μm to several tens μm. Also, the CNT has a very large specific-surface area. As a result, the CNT is as advanced new materials, which will lead the twenty-first century, in the field of next-generation information electronic materials, high-efficiency energy materials, high-functional complex materials, eco-friendly materials, and the like.
However, in spite of various advantages possessed by the CNT, since the CNT has very large agglomeration phenomenon and very large hydrophobic properties, the CNT is poor in terms of the mixed property with other media. That is it does not have solubility with organic solvents or water. Therefore, in order to expand the applications of the CNT while having the advantages of the CNT, a method capable of increasing compatibility with various media and making dispersion efficiency good is needed. As a technology of increasing the compatibility of CNT, there is a functional group substituting technology capable of providing separate characteristics on a surface, for example, there is a method for increasing the specific-surface area of CNT using strong bases such as potassium hydroxide, sodium hydroxide, etc., under vacuum and inert gas atmosphere as described in KR Patent No. 450,029 and a method for functionalizing a CNT using strong acids or strong bases as described in KR Patent Publication Nos. 2001-102598, 2005-9711, and 2007-114553.
However, since the above technologies use strong acids, such as nitric acid, sulfuric acid, etc., or strong bases, such as potassium hydroxide, sodium hydroxide, etc., they are harmful to environment, are not easy to handle, and can cause the corrosion of a reactor. Further, these technologies need further processes, such as a process of washing used acids and bases, which can cause a large amount of harmful wastes. In addition, since they have long reaction times and limited throughput, the efficiency is low. Also and in order to provide the functional group in addition to oxygen on the surface, they need separate processes that consume time and cost.
In addition, K.R. Patent Publication No. 2005-16640 disclosed a method for preparing a carbon nanotube or a nitrogen-added carbon nanotube by pyrolyzing liquid in a reaction chamber, the liquid including a liquid hydrocarbon precursor of one or more carbon, a liquid compound precursor of carbon atom, nitrogen atom, and optionally hydrogen atom and/or one or more carbon and nitrogen consisting of atoms of other chemical elements such as oxygen, and a metal compound precursor of optionally one or more catalytic metal. However, when preparing the nitrogen-added carbon nanotube, the metal compound precursor is included as a catalyst, such that the catalyst treatment after the reaction is troublesome and it is troublesome to perform the reaction by introducing a specific apparatus due to a use of a needle type valve.